1. Field of the Invention
The embodiments herein generally relate to controlled collapse chip connections (C4) in the packaging of semiconductor chips. In particular, the embodiments herein relate to a structure and method of manufacturing a solder ball contact that includes a solder ball, an under bump metallurgy (UBM) structure that underlies the solder ball and is of a greater diameter than the solder ball, and a nitride layer that overlies a portion of the UBM structure and delimits a diameter of the solder ball.
2. Description of Related Art
In controlled collapse chip connection (C4) methodology, solder ball contacts comprise solder bumps or balls that are formed on under bump metallurgy (UBM) structures, which are formed on a wafer of integrated circuit (IC) chips. The UBM structure includes a stack of metal layers that are connected to an underlying metal pad, formed on an upper surface of an IC chip. Ideally, the UBM structure should provide good adhesion to the underlying IC chip through the back-end-of-line (BEOL) processes, including chip attachment to the packaging laminate.
Referring to FIG. 1, a conventional solder ball contact comprises a solder ball 160 and a UBM structure 130. The UBM structure 130 is formed on a bottom and sides of a via 150, disposed within an insulating layer 120, and on a portion of an upper surface of the insulating layer 120 that surrounds the via 150. The via 150 contacts a metal pad 110 that is formed on a topmost layer of an IC chip 105 from which a passivation layer has been removed. The metal pad 110, in turn, contacts an underlying metallization layer or a through-silicon-via of the IC chip 105.
The UBM structure 130 comprises: a lowermost titanium-tungsten (TiW) base layer 132 that provides good adhesion to the insulating layer 120; an overlying copper (Cu) seed layer 134 that forms an efficient electrical conductor; and an uppermost nickel (Ni) barrier layer 136 that separates the Cu seed layer from the solder ball 160. As known in the art, a solder contact can be formed that fills the via 150, extends above the Ni barrier layer 136, and extends outwardly to cover the UBM structure 130. The solder contact is then reflowed to form a solder ball 160, which has a diameter roughly equal to that of the UBM structure 130.
As shown in the related art, an edge of a solder ball needs not overlie a peripheral edge of the underlying UBM structure. Referring to FIGS. 2A-D, a UBM structure and a solder ball are formed by selectively depositing solder on exposed portions of a planar metal stack through a hole in a second insulating layer. A second insulating layer 260, e.g., a polyimide, is deposited over a first insulating layer 220 and over planar metal stacks 250, which are connected by vias 240 to a next-to-last layers of metallurgy 230 of an underlying IC chip, as shown in FIG. 2A. The planar metal stacks 250 contain a zirconium (Zr) base layer, a Ni layer formed on the Zr base layer, a Cu layer formed on Ni layer, a gold (Au) layer formed on the Cu layer, and a topmost lead (Pb) layer. A patterned photoresist layer 270 is deposited over the second insulating layer 260, and holes are then formed through the second insulating layer 260 to the underlying planar metal stacks 250, forming through holes 280, as shown in FIG. 2B. The photoresist layer 270 is then removed. In an embodiment of the conventional method, lead-tin (Pb—Sn) solder, bismuth (Bi) solder, or other solder is selectively deposited on exposed portions of the planar metal stacks 250, forming solder contacts 200 in the through holes of the second insulating layer 260 by immersing the IC wafer in molten solder, FIG. 2C. The molten solder selectively attaches to the exposed portions of the planar metal stacks 250 upon contact to form solder contacts 200. In another embodiment of the conventional method, chemical vapor depositions of the metal species of the solder are preferentially absorbed on the exposed portions of the planar metal stacks 250 to form the solder contacts 200. With either conventional method, the solder contacts 200 may then be reflowed, as is known in the art, to form solder balls 210, as shown in FIG. 2D.
There remains a need for a solder ball contact and a method of manufacturing the solder ball contact that reduces delamination failures between the IC chip and packaging laminate during the back-end-of-line (BEOL) processes.